Ha ha ha.  I had a Cherry Bomb Turbo on an old car more than a decade ago!  They still sell 'em... and they're just the same.  They definately make a 4 cyl sound throaty.  I've seen them online for as low as $25.
 
If you want "loud", simply swapping out the muffler is going to be the first place to start... shy of just cutting stuff off.
 
I will tell you though, I've read many, many people on this board say that exhaust systems don't do much for the 6 cyl.  I could go on and on about the intake, exhaust, and motor mods my straight 6 TJ has... but it doesn't sound agressive at all.  It makes a little noise, but there's just something about this motor that doesn't "sound mean".
 
For a minimal investment, try it out.  See if you like it.  Keep your stock one hanging around, though.  In my opinion, there's better things to spend your dollars on.
 
-X